Changing Permalink type mid blog — Trouble?

Now, I would like to change, from here forward, to blogname.com/custom-name-for-post.

If I make that change immediately in Options/Permalinks will that cause any problems with the earlier 150-some posts that are already named blogname/p=number?

What I’m concerned about is that I don’t want the posts that have the default-style names to lose those — since they’re already Google indexed. I just want posts from here on out to take on the new style.

Will I accomplish that if I change the style in Options/Permalinks now?

You will be fine. All posts are always accessible with blogname.com/?p=number even if you have nice permalinks enabled. Where you will have troubles is if you change your nice permalinks at a later date (from blogname/something/nice to blogname/something/different/to/before).

I think I’m getting that I need to put only /slug in the input box instead of the full path as I did before. But what magic word exactly needs to replace “slug” in the example above in order for WordPress to know to use the smart slugs?